Ross Lake is a large reservoir in the North Cascade mountains of northern Washington state, United States, and southwestern British Columbia, Canada. The lake runs approximately north-south, is 23 miles (37 km) long, up to 1.5 miles (2.5 km) wide, and the full reservoir elevation is 1,604 feet above sea level (489 m).
The U.S. portion of Ross Lake is in the Ross Lake National Recreation Area. North Cascades National Park is adjacent to the west and south, while the Pasayten Wilderness is east of the lake. In British Columbia the lake is a part of Skagit Valley Provincial Park, and Manning Provincial Park is nearby to the east.